The aim of the proposed project is to provide evidence in support of a new concept. The concept states that the preimplantation embryo (PIE) has the capacity to synthesize steroid hormones and that these "PIE steroids" are critical for the maintenance of early pregnancy. We will investigate (1) the presence of various steroid hormones in the PIE, (2) the PIE's capacity to synthesize steroid hormones, and (3) the effects of "PIE steroids" on implantation and various preimplantation phenomena such as morula to blastocyst transformation, shedding and dissolution of the sona pellucida, and various metabolic activities in the PIE. BIBLIOGRAPHIC REFERENCES: Dickmann, Z. (1975) Steroidogenesis in preimplantation embryos. Res. in Reprod. 7 (1) 3. Dickmann, Z., Dey, S.K. and Sen Gupta, J. (1976) A new concept: control of early pregnancy by steroid hormones originating in the preimplantation embryo. Vitamins and Hormones 34: (in press).